1. Field of the Invention
This invention relates to output rectifier circuits of switching mode power supply, and more particularly to multiple outputs converter with synchronous rectifier and diode rectifier applied simultaneously.
2. Description of the Prior Art
DC/DC converters are commonly used to provide power for electronic devices such as computers, communication devices and personal digital assistants. A DC/DC converter converts a DC input voltage to a conditioned DC output voltage and then provides one or multiple output DC voltages. For example, for personal computer and networking applications, a DC/DC converter may be employed to convert a DC input voltage to provide three main output voltages (12V DC, 5V DC and 3.3V DC).
FIG. 1 illustrates a conventional multiple outputs forward converter. The forward converter includes the main power switch 20 coupled to the primary winding 10 of a transformer. In the illustrated embodiment, the main power switch is typically a power switching MOSFET. The forward converter has a first and second output channels to provide a first output voltage Vo1 and a second output voltage Vo2, respectively. Each of the two output channels includes the secondary winding (12, 14), the diode rectifier (30, 32 and 34, 36), an output choke (50, 52) and an output capacitor (60, 62). The two output chokes 50 and 52 are coupled with each other in one core for improving voltage cross regulation and achieving low cost design. The weighted voltage regulator 70 is provided to control the duty cycle of the main switch 20 and to regulate the first and second output voltages Vo1, Vo2.
One drawback of the forward converter in FIG. 1, however, is that the secondary windings 12, 14 and the output chokes 50, 52 could not be ideally coupled without leakage. Moreover, the forward drop voltages of the rectifier diodes decrease when their forward currents decrease. All these factors worsen the cross regulation, especially when one output channel is lightly loaded and the other one is full loaded.
An improvement to the forward convert in FIG. 1 is show in FIG. 2. In this circuit, the secondary windings 12 and 14 are stacked with each other. Compared with that shown in FIG. 1, the coupling of the windings, winding 14 and winding 12 plus winding 14 for the first and second output voltage Vo1 and Vo2 respectively, is well increased. And this benefits the cross regulation of the first and second output voltage Vo1 and Vo2.
FIG. 3 shows the stacked rectifiers for further improving the cross regulation, wherein the forward voltage drop of diodes 34 and 36 of the first output channel are reflected to the second output channel.
However, in these aforementioned circuits, the operation mode of the output chokes 50 and 52 influences the cross regulation of the converter. The cross regulation can be well satisfied only under continuous conduction mode (CCM).
With everlasting trend for lower output voltage requirement, synchronous rectifier has been more widely applied as secondary rectifier for high efficiency operating design. FIG. 4 shows a forward converter, in which the first output channel 501 employs a synchronous rectifier having MOSFETs 40 and 44, and the second output channel 502 employs a diode rectifier having the forward diode 30 and the freewheeling diode 32. The driver 80 is provided to drive the MOSFETs 40 and 44. Due to much lower conduction voltage drop of synchronous rectifier compared with that of diode rectifier, and due to the operation of continuous conduction mode (CCM) of the first output channel 501, there will be a severe cross regulation issue, resulting from different operating conditions of the two output channels 501, 502 at the worsen case of light load operation.